1. Field of the Invention
The invention relates to a device for raising the frequency of electromagnetic radiation, comprising a diode laser for generating the radiation, a non-linear optical medium having a first optical waveguide for guiding a fundamental wave of the diode laser radiation while forming a second harmonic wave, and a wavelength stabilizer for stabilizing the wavelength of the fundamental wave.
The invention also relates to an optical scanning device comprising such a device.
2. Discussion of the Related Art
A device for raising the frequency of electromagnetic radiation of the type described in the opening paragraph is known from the article "Stable blue second-harmonic generation in a KTP waveguide with a diode laser in an external cavity" by F. Laurell in Electronics Letters, September 1993, vol. 29, no. 18, pp. 1629-1630.
Since most non-linear optical materials used for raising the frequency have, however, a relatively small acceptance bandwidth imposing relatively stringent requirements on the diode laser, a relatively small number of diode lasers is readily suitable for this application. Acceptance bandwidth is understood to mean the width of the wavelength band, around a nominal wavelength, of the radiation which is efficiently raised in frequency by the non-linear optical material. The paramount requirements imposed on diode lasers for such applications are:
1) the wavelength band of the radiation emitted by the diode laser should be within the acceptance bandwidth of the non-linear optical material. This requirement limits the number of usable lasers; PA0 2) the emission wavelength of the diode laser should be very stable with respect to the acceptance bandwidth, so that this wavelength is always within the acceptance bandwidth of the non-linear optical material.
In the device described in said article the wavelength of the diode laser is stabilized at a given value by means of an external resonant cavity which is provided with a grating. By varying the orientation of the grating, a different wavelength will be fed back to the diode laser. In this way the wavelength of the diode laser can be adapted to the wavelength at which phase matching occurs in the non-linear optical material.
A drawback of the known device is, however, that the variation of the orientation of the grating is to be effected in a relatively accurate way, because the difference in effective refractive index between the situations where there is phase matching and where there is no phase matching is relatively small. Moreover, the orientation of the grating is varied mechanically, which is a relatively slow process as compared with an electrically controlled variation. To achieve a sufficiently high spectral resolution, the grating (for example, a standard grating having 600 lines/mm) should have a diameter of the order of 5 mm, which limits the compactness of the device.